Injection Molding Goes High Volume

New processes and practices in injection molding are adapting the technology for the high-volume production of components for industrial, automotive, aerospace, and medical applications. These techniques include changes in materials and processes, such as shifting from steel to aluminum tooling, as well as approaches for converting metal part designs into parts that can be realized in plastic.

One shift to help boost volume and reduce costs is the trend toward integrating multiple steps within the mold. Traditionally, after plastic parts come out of the mold, they go through several secondary processes like finishing and assembly. These add cost to the part due to handling, and the need for more equipment and floor space.

For the last few years, Trexel has been doing more inside the molding machine, Steve Braig, the company’s president and CEO, told us.

The award-winning Ford Escape instrument panel design has a complex geometry with changing wall thicknesses, making it difficult to meet required mechanical properties using solid injection molding processes. Creating the panel in the MuCell microcellular foam process reduced weight by more than 1 lb, improved mechanical properties, reduced cycle time, and lowered the part’s cost by $3 per car.

For example, what was previously printed or hot-stamped outside the mold is now finished with an applique or another coat. “Decorating a part in-mold is not without challenges,” Braig said in an interview. “High injection pressure near the applique or label can produce enough force to move it, or cause an ink washout by moving the polymer against the label.”

Trexel’s MuCell process uses lower pressure, avoiding both of those problems, he said. It also saves weight and cycle time, since cavity pressure is about 60 to 70 percent lower than conventional injection molding, requiring less clamp force.

Another trend that reduces cycle time is the shift to aluminum tooling. Although steel tooling is stronger, aluminum heats up and cools down much faster. Aluminum molds are cheaper to make and weigh less, making them easier on process equipment, said Darcy King, president of Unique Tool & Gauge. He went on to say:

With the competitiveness in manufacturing today, especially in automotive, the average cycle time savings of 40 to 50 percent of aluminum over conventional steel molds gives big advantages. That percentage can be achieved when using the design practices we incorporate, and the correct process parameters to optimize the use of aluminum molds.

Unique does several things to make sure tools can last the several-years-long life cycle of an automotive program, including designing them more robustly than a steel tool. For example, for a mold aimed at high-volume applications in millions of parts over a life cycle, the company uses harder, stronger aluminum alloys with excellent wear properties.

Aluminum molds can also shorten lead times. “Our longest lead time is six weeks, versus 12 to 20 weeks using steel,” David Myers, vice president of sales and marketing for DRS Industries, said in an interview. “This lets customers fine-tune the design, or bring in modifications. Tooling cost is also lower, at minimum 20 percent less, or even 30 to 40 percent less the cost model of steel equipment, depending on machining and mold construction.”

Because steel’s conductivity is so poor, there can be variances of 15 to 25 degrees between different areas of the tool, said King. But aluminum’s better conductivity lets core and cavity temperatures remain more consistent, usually within one degree. That stresses the part less, letting it solidify with less warpage and movement, improving its quality.

Since most of my working in the more recent past has been for smaller companies, the motivation is much closer to home, since we need to get any product right the very first time, or else we don't make any profit on it. That is some real motivation, as you can imagine.

Well william, you are Designing For Manufacture already then. I can tell you, there are designers and even engineers who are not. At least not for efficient manufacture. And as the addage of many ways to skin a cat goes, There are many ways to design a product for the same manufacture process. Just because the manufacture dictates how it is designed, doesn't necessarily follow that that is the most efficient design for manufacture. How long does it take to fabricate for example, does it require as many screw? Does it require screws at all? Do the parts require turning over to fit together on the assembly line? If so can they be designed to reduce the amount of turns? Are there sufficient guides in place to ease fitting parts together? Does the design require a certain finish, if not, can it be sparked for easier.quicker removal from the mould? How many other products can we incorporate parts from one or more common moulds? Etc...

As you've already demonstrated with your own experience, this kind of thinking in the design phase is the way forwards for good design engineers, yet not everybody leaving university is leaving with this drummed into their heads.

jrryan , an interesting comparison to cooking. But if I don't have the ingredients for curry, then instead of making a deffective curry I would head in another direction and possibly make some fried chicken or beef stew.

My point is that unless one is intendingg to lead the organization in a new direction, it is a requirement to consider the production capabilities during the design stage, long before checking happens. Not only considering processes available, but also accuracy levels and the cost of those accuracy levels. Ultimately it equates to designing for high yield, hopefully 100%. That can only happen if one keeps production in mind at all times.

As for some of those poorly done injection molded parts with sink marks? YES, I have seen a few of them, and mostly the sink marks are in places where appearance does not matter much. I agree that sink marks are a production flaw, but sometimes they don't affect yield.

Of course, it is not certain that every engineer would also understand the ability of their organizations production department, but it is certain that at least some part of a design team should have a good grasp of how the product would be made. For many years I have asked other engineers, as we were discussing a design, "How would they make that?", and on quite a few occasions the designer had to visit the production people and find out. I have saved companies a few dollars that way, on occasion. It turns out that there are a few things that can be designed but that can not be produced, at least, not economoically.

P.S. this link contains some interesting further reading on aluminium tooling for injection moulding: http://www.phoenixproto.com/about/aluminum-tooling-information/aluminum-tooling-myths/ as expected 7075 and QC-10 are in there, along with a few other variants.

William, I think the analogy you used is apt, and I definitely welcome the prevalence of idea that good design is by definition design for manufacture, but that precludes the fact that there are so many poorly designed products in the marketplace. How many times have you seen an injection moulded product with significant sinking? or another with a level of fabrication that clearly could be vastly simplified with snapfits?

So, to take your analogy a little bit further, if you were making a meal out of the ingredients in your cupboard, and you were intending to create a curry, but only had salt, pepper, tumeric, milk and chicken, you would do the best you could with the ingredients that matched the recipe. But what if you had other ingredients that don't normally feature in a curry, like bicarbonate of soda, or vinegar, or butter? They aren't on the list, so you overlook how they could be used in your best-effort "design". If you had the time and inclination, you could research how these other ingredients could produce much more vibrant flavour combinations and thus produce a better assimilation of the real thing.

But that still isn't really analogous of the DFM issue, To be a proper analogy, not only would the ingredients have to be throughly explored for suitability, the cooking of the dinner would have to be streamlined for bulk output, so you'd figure out your prep times, brebatch certain ingredients, cook everything in one pot instead of four, and steam your rice in a double boiler over the top. These are all simple efficiency tweaks, and it is merely another form of tweak that brings the concern for efficiency into the design process instead of the re-design process where mistakes and time wasted are corrected after the fact or through manufacturing hacks on the production line.

If you are a good designer, are you already implementing DFM? Well most likely yes, but it is possible that you aren't and the DFM takes place at the design checking stage, where people with more experience of manufacturing provide their input, but if you are a bad designer, then you definitely aren't taking any consideration of the manufacturing process (or at least as little as is necessary to develop a product) and that means a lot of wasted time, money, resources and ultimately really poor, crappy products that are nothing more than future landfill.

I agree with William--when I first heard of DFM, my initial reaction was--"as opposed to what? Design Not For Manufacturing? Design Without Manufacturing?" DFT made sense, and later, DFR (R = either reassembly or recycling). OTOH, manufacturing processes, especially on highly automated lines, have gotten highly complex, as have some products, so more tailored DFM makes sense.

I don't make any claim of originality about the assertions in my previous posting, nor that the concepts presented are that new. In fact, my intended point was "how else could you do it? The idea of keepingproduction isolated from design and engineering has always been a poor choice. At least, I think that we are all aware that it is a poor choice.

University of Southampton researchers have come up with a way to 3D print transparent optical fibers like those used in fiber-optic telecommunications cables, potentially boosting frequency and reducing loss.

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